Optional (:)

Optional[pattdefault] or patt : default
is a pattern which matches patt and which, if omitted should be replaced by default.

Note that symb : patt represents a Pattern object. However, there is no disambiguity, since symb has to be a symbol in this case.

s_. is equivalent to Optional[s_] and represents an optional parameter which, if omitted, gets its value from Default.